1. Field of the Invention
This invention pertains to a method for forming a cylindrical photosensitive element and, in particular, a method for forming a seamless cylindrical photopolymerizable element suitable for use as a flexographic printing element.
2. Description of Related Art
Photopolymerizable materials can be formed into sheets or layers by several known methods such as solvent casting, hot pressing, calendering and extrusion. A preferred method of forming photopolymerizable materials for use as flexographic printing elements is by extrusion calendering the photopolymerizable material. In extrusion calendering, the printing element is prepared by passing a mass of hot photopolymerizable material into an extrusion die forming a layer, passing the layer into the nip of a calender and, while still hot, calendering the photopolymerizable material between two flat surfaces, generally two flexible films, to form a multilayer web. The films can include multiple layers or compound films. A film bearing a thin layer of flexible, polymeric film is an example of a compound film. After extrusion and calendering at elevated temperatures, the web is held in tension in the machine direction by a pair of nip rollers while the multilayer web is cooled, for example, with blown air. The printing element as a multilayer web can be cut into suitable size sheets. Extrusion and calendering of a polymeric compositions are disclosed, for example, in Gruetzmacher et al., U.S. Pat. No. 4,427,759; and in Min, U.S. Pat. No. 4,622,088.
Although typically photopolymerizable printing elements are used in sheet form, there are particular applications and advantages to using the printing element in a continuous cylindrical form. Continuous printing elements have applications in the flexographic printing of continuous designs such as in wallpaper, decoration and gift wrapping paper, and tight-fit conditions for registration, since the designs can be easily printed without print-through of the plate seam. Furthermore, such continuous printing elements are well-suited for mounting on laser exposure equipment where it can replace the drum or be mounted on the drum for exposure by a laser to achieve precise registration.
The formation of seamless, continuous printing elements can be accomplished by several methods. The photopolymerizable flat sheet elements can be reprocessed by wrapping the element around a cylindrical form, usually a printing sleeve or the printing cylinder itself, and fusing or joining the edges together to form a seamless, continuous element. Processes for joining the edges of a plate into a cylindrical form have been disclosed, for example, in German patent DE 28 44 426, United Kingdom patent GB 1 579 817, European patent application EP 0 469 375, U.S. Pat. No. 4,883,742, and U.S. Pat. No. 4,871,650. A problem with the prior methods of joining the edges to form the continuous cylinder is that the results of printing with the joined edge element are often unsatisfactory, particularly when the joined edge falls within the effective printing area of the plate. Although the prior art often describes the so-formed continuous element as xe2x80x9cseamlessxe2x80x9d, the joined seam has not completely formed a continuum of the photopolymerizable layer at the edges since the joined seam is visible in and interrupts the printed image.
U.S. Pat. No. 5,798,019 discloses an apparatus and a process for forming a cylindrical photosensitive element of uniform thickness on a flexible sleeve without sanding, grinding or additional polishing steps. The method involves supplying a stream of molten photopolymerizable material onto the sleeve supported directly on a mandrel, calendering the molten photopolymerizable material to have a substantially constant thickness on the sleeve, moving the sleeve around and along the mandrel in a helical fashion to polish an outer surface of the element, and during the calendering step, heating the photopolymerizable material. In a first mode, the flexible sleeve is mounted on an air lubricated mandrel such that the air permits the rotational and axial movement of the flexible sleeve with preferably at least two calender rolls positioned around the sleeve at a predefined clearance, approximately equal to the desired thickness of the photopolymerizable material on the sleeve. The stream of molten photopolymerizable material is fed into a gap between the sleeve and a calender roll. Once there is coverage of the photopolymerizable material around the circumference of the sleeve, the contact of the photopolymerizable material with the calender rolls will rotate the sleeve. At the same time, the sleeve is pushed laterally along the mandrel causing the polymerizable material to be wrapped spirally around the sleeve. The length of the resulting cylindrical photosensitive element is longer than the calendering roll/s. In a second mode, the flexible sleeve is mounted onto a mandrel with the calender rolls positioned around the sleeve at the predefined clearance. A solid sheet of photopolymerizable material is fed into the gap between the sleeve and the calender roll. Once there is coverage of the photopolymerizable material around the circumference of the sleeve, the contact of the photopolymerizable material with the calender rolls will rotate the sleeve and mandrel together. In this mode, the calender rolls are the same length as or longer than the cylindrical element and thus there is no lateral movement of the sleeve along the mandrel.
However, the process of U.S. Pat. No. 5,798,019 is limited in its ability to produce a cylindrical photopolymerizable element on a flexible polymeric sleeve with acceptable dimensional uniformity in the mode where the sleeve rotates about the mandrel. The polymeric sleeve may also move laterally along the mandrel. It is desirable for the cylindrical photopolymerizable element to be made with a flexible polymeric sleeve because the sleeve is transparent to UV light, and therefore the element can be exposed through the back (i.e., the sleeve) to establish a floor thickness in the printing element. For example, the polymeric sleeve can be formed from polyester film, such as the sleeve disclosed by McConnell in U.S. Pat. No. 5,301,610. A problem arises during the formation process in that the flexible polymeric sleeve distorts uncontrollably and is not of uniform size and/or shape. In order to reduce the force required to both rotate and translate the flexible sleeve along the mandrel, the interface between the sleeve and the mandrel is lubricated using 5 to 60 psig compressed air. At the same time, the sleeve is at or near the elevated temperature associated with the molten photopolymer, and thus the air pressure between the polymeric sleeve and the mandrel causes the sleeve to balloon and permanently deform. Additionally, near the leading and trailing ends of the sleeve where the air pressure rapidly decreases to atmospheric pressure, the hot photopolymer causes the sleeve to shrink, thereby reducing the internal diameter of the sleeve at the ends. Distortion or non-uniformity of the sleeve will result in unacceptable interference fit of the cylindrical printing element with the printing cylinder. It is an -objective of the present invention to provide a method for forming a cylindrical photosensitive printing element having acceptable dimensional uniformity.
A method for forming a seamless cylindrical photosensitive element on a flexible cylindrical sleeve comprises:
supporting the sleeve on a substantially rigid and thermally stable sleeve support such that the sleeve and sleeve support act as a unitary structure, the sleeve support having a longitudinal axis and adapted to be mounted onto a mandrel;
supplying a substantially molten stream of photopolymerizable material onto the sleeve;
providing relative axial movement between the sleeve support and the supply of photopolymerizable material along the longitudinal axis of the sleeve support;
calendering the molten photopolymerizable material on the sleeve by metering with at least one rotating calender roll the photopolymerizable material to have a substantially constant thickness on the sleeve and rotating the sleeve support to polish an outer circumferential surface of the material to a seamless uniform state without sanding, grinding or additional polishing steps, thereby forming the seamless cylindrical photosensitive element; and
during the calendering step, heating the photopolymerizable material.
In accordance with this invention, there is provided an intermediate element for making a seamless cylindrical printing element comprising:
a) a hollow cylindrical sleeve of at least one polymeric film mounted onto a substantially rigid and thermally stable sleeve support for maintaining the sleeve in a substantially cylindrical shape, the sleeve support adapted to be mounted onto a mandrel;
b) at least one seamless layer of a photopolymerizable material on an exterior surface of the sleeve; and
c) optionally, an adhesive layer disposed between the sleeve and the at least one seamless layer or an exterior surface of the sleeve having an adhesion promoting surface.